Circuit pack layout with improved dissipation of heat produced by high power electronic components

ABSTRACT

Dissipation of the heat produced by the operation of electronic circuitry may be improved by a heat sink which comprises a flat base from which a number of vertical fins extend. The fins are parallel to one another and define a number of parallel channels into which coolant flow is directed. The thermal resistance of the heat sink is optimized by setting fin thickness and channel width parameters to appropriate values. The heat sink may be attached in a heat conductive manner to a heat producing electronic component. One or more of these heat sinked components may be laid out in an in-line or staggered arrangement on a support in the form of a circuit pack. Cooling fluid is delivered to the circuit pack in a variety of ways to cool the heat sinked components. A method of determining the optimum fin thickness and channel width parameters involves determining a relationship between total thermal resistance of the heat sink and combinations of fin thickness and channel width parameters. A contour plot is produced in accordance with the relationship referred to above. The contour plot shows regions of optimum heat dissipation for heat sinks in accordance with the geometry identified here.

CROSS REFERENCE TO RELATED APPLICATION

This application is related to U.S. application Ser. No. [Azar 6] ofAzar, entitled "Narrow Channel Finned Heat Sinking For Cooling HighPower Electronic Components," filed in the U.S. Patent and TrademarkOffice on the same day this application was filed in the U.S. Patent andTrademark Office.

FIELD OF THE INVENTION

This invention relates to the cooling of heat-producing electroniccomponents. More particularly, this invention relates to heat sinks,coolant delivery systems, circuit layouts, and methods of optimizing thedimensions of heat sinks.

BACKGROUND OF THE INVENTION

Effectively dissipating the heat produced by the operation of electroniccomponents is an important concern in optimizing the performance of thecircuitry in which those components are used. In addition to optimizingperformance, effective heat dissipation also helps to prolong the usefullife of those components. Heat dissipation is particularly important inthe case of high power electronic components which may produce threewatts per square centimeter or more. Some exotic methods of cooling ofhigh power electronic components, such as forced liquid cooling of heatsinks attached to those components, have been proposed, but thesemethods are not desirable because they are costly to implement andmaintain. Simple air cooling techniques have been avoided because of theinadequate performance of heat sinks and coolant delivery systemsdeveloped to date and because of a general perception that air coolingis not up to the task of adequately dissipating the heat produced bytoday's high power electronic components.

There has been some work involving the use of air cooled narrow channeland microchannel heat sinks to cool electronic components. For example,Goldberg, "Narrow Channel Forced Air Heat Sink," IEEE Transactions onComponents, Hybrids, and Manufacturing Technology, Vol. CHMT-7, No. 1,March 1984, pp. 154-159 refers to confined channel heat sinks in whichchannel spacing and width were either 0.0127 cm., 0.0254 cm. or 0.0635cm. Air was used as the cooling fluid and it was ducted directly to theheat sink. The heat sink of Goldberg was said to have achieved thermalimpedances of 3.4° to 5.9° C. per watt.

Hilbert et al., "High Performance Micro Channel Air Cooling,"Proceedings of the Sixth Annual IEEE SEMI-THERM Symposium, pp. 108-113)1990, refers to an array of microchannel finned heat sinks in which airwas specially ducted to the top of each heat sink. The heat sinks weresaid to have achieved thermal impedances in the range of 1.6° to 2.1° C.per watt.

Both Goldberg and Hilbert do not deal with heat sinks having an optimumconfiguration which maximizes their heat dissipation capability. Inaddition, both Goldberg and Hilbert refer to unusually shaped heat sinkswhich are difficult and expensive to manufacture. Also, Goldberg andHilbert achieve the performance they achieve with coolant deliverysystems which are non-standard in most electronic systems. Mostelectronic systems either push or pull air flow across electroniccomponents which are situated on flat circuit boards which are disposedin a parallel configuration in bays or racks in a cabinet. There is afan shelf placed at the top or bottom of the system to force air intothe cabinet and through the spaces between the parallel circuit cards.The flow impingement techniques of Goldberg and Hilbert cannot be usedwith, or readily adapted to; these kinds of cooling arrangements.

Efforts such as those of Goldberg and Hilbert, therefore, really havenot satisfied a long felt need to provide a cooling system forelectronic components which is able to most effectively dissipate theheat generated by today's high power electronic components using simpleair cooling technology without drastic change of the mechanicalarrangement of the components.

SUMMARY OF THE INVENTION

The need for adequate heat dissipation capabilities noted above issatisfied by a variety of novel fluid cooled circuit packconfigurations. The circuit packs comprise a plurality of electroniccomponents attached to optimized heat sinks which are arranged in apredetermined pattern on a flat base plate. Examples of suitablearrangements for the electronic components and associated heat sinksinclude an in-line configuration and a staggered configuration ofcomponents and heat sinks. Cooling fluid may be drawn across the heatsinks by a fan which pulls coolant through either an end plate of ahousing enclosing the circuit pack or through a plurality of slots in atop plate of the circuit pack housing situated over the components andheat sinks.

Novel open channel heat sinks which may be used with the circuit packsof this invention are dimensioned in a fashion which optimizes the heatdissipation capability of the heat sink. The heat sink comprises aplurality of fins, each having a predetermined thickness and apredetermined height. A plurality of channels each having apredetermined width are located between the fins of the heat sink. Theheat dissipation capability of the heat sink can be optimized if finthickness and channel width parameters are jointly and appropriatelydetermined. For example, the heat dissipation capability of the heatsink is optimized if the ratio of the fin thickness to the fin height isabout 0.005 to about 0.055 and the ratio of the channel width to the finheight is about 0.030 to about 0.130.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an example of a narrow channel heat sink in accordance withthis invention.

FIG. 2 shows an example of a circuit pack in accordance with thisinvention having a staggered configuration of heat sinked electroniccomponents.

FIG. 3 shows an example of a circuit pack in accordance with thisinvention having an in-line configuration of heat sinked electroniccomponents.

FIG. 4 shows an example of a circuit pack in accordance with thisinvention having a plurality of rows of staggered heat sinkedcomponents.

FIG. 5 shows a coolant delivery apparatus for cooling the circuit packsshown in any of FIGS. 2 through 4.

FIG. 6 is a top view of a portion of the apparatus shown in FIG. 5illustrating air entry slots and pressure taps in the top plate of anenclosure for the circuit pack.

FIG. 7 shows pressure drop measurements with respect to ambient and withrespect to channel input for a circuit pack with a staggered heat sinkedcomponent configuration.

FIGS. 8 and 9 illustrate temperature rises above ambient for eachcomponent in a circuit pack with a slotted cover plate for two differentfan voltages.

FIG. 10 is a comparison between the performances of circuit packs havinga slotted cover plate and a solid cover plate.

FIG. 11 is a comparison of the effect of circuit pack layout and airflow entry method on thermal impedance.

FIGS. 12, 13, 14, and 15 are contour plots illustrating the optimizationof heat sink dimension at four different pressure drops across the heatsink.

FIG. 16 shows an example of a heat sink in accordance with thisinvention into which an electronic heat producing component isintegrated.

FIG. 17 shows a heat sink in accordance with this invention which hasbeen incorporated into the structure of a molded or encapsulatedelectronic heat producing component.

DETAILED DESCRIPTION

FIG. 1 shows an example of an optimized heat sink 10 in accordance withthis invention. The heat sink 10 comprises a row of fins 12, one ofwhich explicitly bears the reference numeral 12 in FIG. 1. Each fin hasa predetermined height H and a predetermined thickness c. The height Hmay be from about 0.25 cm. to about 3.2 cm. The fins project from arectangularly shaped base 14 of the heat sink 10. The base 14 has apredetermined width W and a predetermined depth D. For example, thewidth of the base 14 may be about 2.5 cm and the depth of the base 14likewise may be about 2.5 cm. The overall height of the heat sink, fromthe bottom of the base 14 to the tips of the fins 12, may be about 3.8cm. to about 7.6 cm. Although the base 14 of the heat sink 10 in theexample of FIG. 1 is rectangular, the base may be any convenient shapesuch as circular, rectangular, or triangular.

The fins 12 comprise rectangularly shaped protrusions standingsubstantially upright on the base 14. The fins 12 extend substantiallyfrom the front to the back of the heat sink and define a plurality ofparallel rectangular channels or groove 16 of predeterminedsubstantially constant width S between adjacent fins 12. As shown inFIG. 1, the channels are open at the top of the heat sink 10. The fins12 have substantially rectangular cross-sections parallel andperpendicular to the longitudinal axes of the channels. In accordancewith the invention and as described in more detail below, the finthickness τ and the channel width S are selected so that the heatdissipation capabilities of the heat sink are optimized.

The heat sink is attached to a heat producing electronic component inany heat conductive manner. For example, the heat sink can be attachedto such a component by means of a heat conductive adhesive, such as aheat conductive epoxy.

The heat sink 10 in FIG. 1 may be made from any suitable heat conductivematerial such as aluminum, copper, or other metallic material. It mayalso be a semiconductor, a ceramic, a composite, or an alloy. Thechannels and fins may be formed in a rectangular block of such heatconductive material by any of a variety of methods. For example, thechannels and the fins may be created by techniques such ascrystal-orientation-dependent etching, precision sawing, electricdischarge machining, or numerically controlled machining.

Another configuration that a heat sink in accordance with this inventionmay take is a folded-fin heat sink similar to those used in airconditioning and automotive radiators. The fabrication technique used tomake folded fin heat sinks can produce fin thicknesses and spacings inthe range of interest at a relatively low cost. In a typicalapplication, aluminum sheet in the desired thickness is folded over in aserpentine fashion so that a desired channel spacing is created. Thisfolded fin may then be bonded to an aluminum base, for example, by dipbrazing. The heat sink can then be attached to an electronic device inany manner which promotes conduction of heat from the device to the heatsink. For example, the device may be attached via some heat conductiveepoxy. The heat sink can also be attached to the electronic component byany other mechanical means which puts the device in intimate heatconductive contact with the heat sink. The folded fin construction hasthe advantage of connecting adjacent fin tips which makes the heat sinkmore resistant to damage from handling. One advantage of using such afolded fin heat sink is that it may be lanced, offset, or wavey to breakup thermal boundary layers which may form in the cooling fluid flow.

It has been found that the heat dissipation performance of a heat sink,such as the heat sink 10 of FIG. 1 or the folded fin heat sink describedabove, can be optimized by appropriately selecting a combination of finthickness and channel width parameters. In particular, it has been foundthat, for a given pressure drop across the heat sink, there is anoptimum range of fin thicknesses and an optimum range of spacingsbetween the fins. The performance of such an optimized heat sink interms of thermal resistance and heat dissipation capability issubstantially improved as compared with prior heat sinks. It has beenfound that this improvement in performance may be achieved if the ratioof the predetermined fin thickness to predetermined fin height (τ/H) isfrom about 0.005 to about 0.055 and the ratio of the predeterminedchannel width to the predetermined fin height (S/H) is from about 0.030to about 0.130 for pressure drops across the heat sink of about 0.05 cmH₂ O to about 1.5 cm H₂ O. The actual ranges within which these ratiosfall may be selected so as to optimize the heat dissipation capabilitiesof the heat sink for some specific fluid pressure drops expected to befound across the heat sink. In one specific example of this invention,the ratio of the fin thickness to the fin height may be from about 0.005to 0.055 and the ratio of the channel width to fin height may be fromabout 0.08 to 0.13 for a pressure drop of about 0.05 cm H₂ O. In asecond example of this invention, the ratio of the fin thickness to thefin height may be from about 0.005 to about 0.055 and the ratio of thechannel width to fin height may be from about 0.060 to about 0.110 for apressure drop of 0.15 cm. H₂ O. In a third example of this invention,the ratio of the fin thickness to fin height may be from about 0.005 to0.055 and the ratio of the channel width to the fin height may be fromabout 0.040 to about 0.090 for a pressure drop of 0.05 cm. H₂ O. Infourth example of this invention, the ratio of the fin thickness to thefin height may be from about 0.005 to 0.055 and the ratio of the channelwidth to fin height may be from about 0.03 to 0.08 for a pressure dropof 1.5 cm H₂ O.

As shown in FIGS. 12-15, there are specific optimum values for thechannel spacing parameter and the fin thickness parameter forrepresentative values of pressure drop across the heat sink. Theseoptimum values are those for which thermal impedance is a minimum andthe heat dissipation is, therefore, a maximum. Those values are:

a.) about 0.103 for the channel spacing parameter S/H and about 0.014for the fin thickness parameter τ/H at a pressure difference ΔP acrossthe heat sink of about 0.05 cm. H₂ O;

b.) about 0.077 for the channel spacing parameter S/H and about 0.014for the fin thickness parameter τ/H at a pressure difference ΔP acrossthe heat sink of about 0.15 cm. H₂ O;

c.) about 0.057 for the channel spacing parameter S/H and about 0.014for the fin thickness parameter τ/H at a pressure difference ΔP acrossthe heat sink of about 0.5 cm. H₂ O; and

d.) about 0.045 for the channel spacing parameter S/H and about 0.015for the fin thickness parameter τ/H for a pressure difference ΔP acrossthe heat sink of about 1.5 cm. H₂ O.

The thermal impedances for the heat sinks of items a.)-d.) above wereabout 2.4° C. per watt, 1.54° C. per watt, 0.94° C. per watt, and 0.61°C./watt, respectively.

In one specific example of a heat sink in accordance with thisinvention, tests of which are described in detail below, the dimensionsof the base 14 of the heat sink comprise a width dimension of 2.5 cm.and a length dimension of 2.5 cm. The total height of the heat sink fromthe bottom of the base 14 to the tips of the fins 12 is about 2.0 cm.The height H of each fin 12 is about 1.25 cm., the thickness τ of eachfin 12 is about 0.4 mm, and the width S of each channel is about 1.1 mm.As a result of actual tests, it has been found that the heat produced byan electronic component attached to the base of such a heat sink is moreeffectively dissipated than is the case with prior heat sinks. Asignificantly reduced temperature rise results from the operation ofelectronic components in a circuit pack containing this heat sink. Thethermal resistances of such a heat sink is substantially reduced ascompared with the thermal resistances of prior heat sinks used undersimilar circumstances.

FIG. 2 shows an example of a layout of elements in a circuit pack inaccordance with this invention. FIG. 2 shows schematically a circuitpack comprising six circuit elements 18, 20, 22, 24, 26, and 28, whichare situated on a base plate 30 which, for example, may be made of aglass-epoxy material. The circuit pack, as is more evident in FIGS. 5and 6, is enclosed in a housing through which cooling fluid is drawn tocool the heat producing electronic components on the base plate 30 ofthe circuit pack. The housing and base plate 30 from a sort of windtunnel which removes the heat produced by those electronic components.In one tested example of the invention, the circuit elements 18, 20, 22,24, 26, and 28 may each comprise an electronic component attached to analuminum heat sink having a 2.5 cm by 2.5 cm by 2.0 cm base 14 with fins12 which are 0.4 mm thick defining parallel channels which are 1.1 mmwide. The channels of the heat sinks are oriented generally parallel tothe direction of fluid flow through the housing indicated by arrows 32.A transistor is one example of an electronic component which may beattached in any heat conductive manner to the base of the heat sink. Asshown in FIG. 2, the heat sinked components are arranged in a rowextending perpendicularly with respect to the direction of air flowindicated by the arrows 32 in FIG. 2. Alternate ones of the heat sinkedcomponents are also staggered in a direction parallel to the directionof fluid flow. The minimum spacing between adjacent heat sinks may beabout 0.254 cm.

FIG. 3 shows an alternative component layout for a circuit pack inaccordance with this invention. The circuit elements 18, 20, 22, 24, 26,and 28 are situated on base plate 30 in an in-line configurationcomprising a number of heat sinked components arranged in a rowextending perpendicularly with respect to the direction of air flowillustrated by the arrows 32. The channels of the heat sinks arearranged generally parallel to the direction of coolant flow indicatedby arrows 32. The heat sinked components are not staggered as they arein FIG. 2.

FIG. 4 is yet another component layout for a circuit pack. FIG. 4 showsan arrangement of heat sinked components, one of which has been given areference numeral 34 in FIG. 4. The arrangement in FIG. 4 may beconsidered a combination of an in-line and staggered configuration ofheat sinked components which are generally uniformly distributed on thebase plate 30 in FIG. 4.

In FIG. 4, there are five rows of heat sinked components. Each rowcomprises two heat sinked components located on a line perpendicular tothe direction of coolant flow indicated by arrows 32. The components ineach row are staggered with respect to the components in adjacent rowsin a direction perpendicular to the direction of coolant flow. This isto ensure that each component is not located in the direct wakes ofadjacent components created by flow of cooling fluid through the circuitpack housing. This arrangement has been found to improve the heatdissipation capabilities of the circuit pack.

FIGS. 5 and 6 illustrate an apparatus for delivery of cooling air to theheat sinked components. FIG. 5 shows a structure 36 which comprises arectangular housing for enclosing a number of heat sinked componentsmounted on a base plate 30 as shown in any one of FIGS. 2 to 4. Theenclosure comprises the aforementioned base plate 30 shown in any ofFIGS. 2 to 4, vertically disposed end plates 38 and 39, and ahorizontally disposed cover plate 40. Two vertically disposed sideplates are situated on the front and back of the FIG. 5 apparatusbetween the base plate 30 and the cover plate 40. The end plates 38 and39, the cover plate 40, and the side plates may be made of a plasticmaterial such as that sold under the LEXAN trademark. The base plate 30,end plates 38 and 39, cover plate 40, and the two side plates define arectangular housing into which the heat sinked components are arrangedas shown in any one of FIGS. 2 to 4. The vertical spacing between thecover plate 40 and the base plate 30 is such that a controlled amount ofspace is defined between the cover plate 40 and the tips of the heatsinks connected to the electronic components. The spacing between thecover plate and tips of the heat sinks may be from about zero to about0.25 cm., for example. Preferably, the spacing between the tips of theheat sinks and the cover plate 40 is made as small as possible. In somepreferred examples of the invention, the cover plate 40 may bepositioned so that there is no clearance between the tips of the heatsink fins and the cover plate 40.

One end of the circuit pack and housing shown in FIG. 5 communicateswith a fan 42 located in a flared structure attached to the end of thatapparatus. Any mechanism which creates a flow of coolant through thecircuit pack and housing may be used instead of the fan 42 and flaredstructure shown in FIG. 5. The fan 42 draws cooling air into one or moreinlet openings in the circuit pack housing, across the heat sinks, andout through one or more openings in the end plate 39 of the circuit packhousing. The outlet openings can be one or more appropriately sizedcircular openings in an end wall 39 of the housing adjacent the fan andopposing the end plate 38. Flow of cooling fluid through the apparatusof FIG. 5 between the cover plate 40 and the base plate 30 dissipatesthe heat generated by the operation of the electronic componentsattached to the heat sinks. The fan 42 exhausts air heated by contactwith the heat sinks as indicated by the arrows 44 shown in FIG. 5.

There are two ways in which cooling air may be introduced through one ormore inlet openings into the circuit pack housing of FIG. 5. Onepreferred way in which air is input to the apparatus of FIG. 5 is by theprovision of a number of air entry slots 46 in the cover plate 40 asindicated in FIG. 6. There preferably is one such air entry slot 46substantially superimposed over each of the heat sinks of the circuitpack. The illustration of the cover plate 40 in FIG. 6 shows anarrangement of air entry slots 46 for the circuit pack layout shown inFIG. 4 which involves one air entry slot 46 superimposed over each ofthe heat sinked components 34 as shown in FIG. 4. FIG. 6 also shows anumber of pressure taps in the cover plate 40 which can be used to makeair pressure measurements in the circuit pack indicated in the testresults described below.

In this example of a circuit pack in accordance with this invention, thefan 42 draws cooling air through each of the air entry slots 46. Thecooling air impinges on the heat sinks directly below each slot 46 in adirection perpendicular to the plane of the base plate 30, and then isdrawn in a horizontal direction parallel to the base plate 30 throughthe channels between the fins of the heat sinks. The cooling air then isexhausted as shown by the arrows 44 in FIG. 5.

In one embodiment of such an air delivery system, involving air entryslots 46 in the cover plate 40, the sizes of the air entry slots 46 maybe made different so that a uniform, balanced coolant flow is maintainedthroughout the circuit pack housing. In particular, the sizes of theslots may be made progressively smaller the farther downstream they arelocated. For example, as shown in FIG. 6, the two slots furthestupstream have length and width dimensions which are approximately thesame as the width and depth dimensions of the heat sinks centered underthose upstream slots 46, namely, about 2.5 cm. by 2.5 cm. in the exampledescribed above. The length dimensions of the slots 46 graduallydecrease the further downstream they are located. The length dimensionof the two slots furthest downstream may be only about 60% of the lengthdimension of the slots furthest upstream in this example of theinvention, namely, 1.5 cm. in the example described above.

An alternative method of delivering cooling air to a circuit pack, suchas the one shown in FIGS. 5 and 6, involves the provision of a solid orunslotted top plate 40 and the removal of the end plate 38. In thiscase, cooling air is drawn through the opening in the end of the circuitpack housing, through the channels of the heat sinks in the circuit packin a direction parallel to the base plate 30, and out the exhaust asindicated by arrows 44 in FIG. 5.

DETAILED EXPERIMENTAL EXAMPLES

An actual heat sink was constructed of a block of aluminum which was 2.5cm. wide by 2.5 cm. long by 2.0 cm. high. Fins 0.4 mm thick weremachined 1.1 mm apart in the top surface of the block of aluminum, asshown in FIG. 1. A transistor was used as the heat source and it wasaffixed to the heat sink in a 1.5 cm.² groove in the base plate of theheat sink. A transistor was used as a heat source because it can producehigh wattage and is moderately compact about (1 cm.²). A circuit packmade of glass-epoxy (27.5 cm.×20 cm.) supported a number of these heatsinked components. The components were placed in the staggered andin-line arrangements of FIGS. 2-4 to investigate the effect of circuitpack layout. The distance between the edges of adjacent heat sinks wasabout 0.127 cm. in FIGS. 2 and 3. The vertical separation betweenalternate rows of components in FIG. 4, for example, the verticaldistance between components numbered 5 and 7 in FIG. 4, was about 6.35cm. The vertical distance between adjacent rows of components in FIG. 4,for example, the vertical distance between the edges of the componentsnumbered 5 and 6 in FIG. 4, was about 2 cm. The horizontal separationbetween the vertical rows of components in FIG. 4, for example, thehorizontal separation between the edges of the components numbered 5 and6 in FIG. 4, was about 1 cm.

Air was introduced to the heat sinks through slots in the top plate orthrough the end wall of the circuit pack housing. An impingementconfiguration was achieved by creating slots, positioned exactly abovethe components, in the top plate of the circuit pack housing. A secondmethod for air flow introduction was arranged by removing the end plateand replacing the top cover with a solid plate. The data reported hereare for both slotted or solid covers.

FIG. 6 shows a circuit pack, housing, and fan acting as a sort of windtunnel for this experiment. The fan used for the experiment was a EG&GRotron (Patriot-DC) fan. A top view of the cover plate is shown in FTG.6 which shows the positions (A, B, C, D, and E) where air pressure wasmeasured. In one case, no clearance between the cover plate and top ofthe heat sinks existed. The circuit pack was then placed inside whatmore resembles a conventional wind tunnel, which simulates a typicalarrangement of electronic equipment in a frame or bay. In this case,clearance existed between the heat sink tips and the cover plate. Thisconventional wind tunnel arrangement was used to investigate the effectof flow bypass on thermal performance of the heat sink.

To determine the thermal performance of a heat sink in accordance withthis invention, several parameters were tracked and varied. Theseincluded air flow rate, component layout, component power dissipation,and tip clearance. Three different air flow rates, representing low tohigh amounts of forced air convection cooling, were used. Because of thegeometry and component layout, the fan voltage and tunnel pressure dropwere recorded as a measure of variation in air flow rate. FIG. 7 showsthe pressure drops for three different fan voltages, 16, 32, and 48volts. These pressure differences are reported with respect to ambient(positive values) and with respect to the inlet of the circuit packhousing (negative values). The air velocity associated with the pressuredrops, measured at the center of the channel, were 2.3, 4.8, and 6.6M/s.

The thermal performance of the heat sink is reported in terms of thetemperature rise over ambient δT and thermal impedance, Θ_(ja). Thetemperature rise used for calculation of thermal impedance is based onthe temperature rise between the heat sink base plate and room ambient.FIGS. 8 and 9 show the δT and Θ_(ja) at different component powerdissipations (13, 16, and 20W) at 16V and 48V fan voltages,respectively. The abscissa shows the component number on the circuitpack. (Those numbers are reproduced in FIGS. 2-4 in addition to thereference numerals already mentioned in the description above.) For thisconfiguration, air is impinged on the component through the slottedcover plate described above. These show that the temperature rise, evenfor a low amount of forced air convection cooling, does not exceed 38°C. and Θ_(ja) a varies from only 0.78 to only 1.910 C/W, for both airvelocities considered. The device junction temperature is well below anindustry standard of 125° C. for operation of the device in a worst caseambient temperature of 50° C.

FIG. 10 shows the impact of different coolant delivery systems on thedissipation of heat produced by the components and the performance ofthe heat sinks in the circuit pack. Two component power dissipations (13and 16W) are reported at a fan voltage of 32V for the slotted and solidcover plates. In the latter, the air flow is parallel to the circuitpack and there is no tip clearance between the heat sink and the coverplate. The figure shows that the air flow direction will impact thetemperature rise. Specifically, improved performance is obtained whendirect impingement through slots in the cover plate is used. However,the cooling achieved by introducing the flow parallel to the circuitpack through an end wall is still significant and certainly withinacceptable design limits. Further, FIG. 10 shows that cooling is afunction of component position on the circuit pack and similar levels ofcooling can be attained with both types of flow introduction. This isdemonstrated by components 5 and 6. One may conclude that exemplarythermal performance of a properly designed narrow channel heat sink inaccordance with this invention may be achieved with either coolantdelivery system.

FIG. 11 further corroborates the conclusion stated above. FIG. 11 showsΘ_(ja) for various flow introduction methods and component arrangements.FIG. 11 also contains data relating to a situation where there isclearance (represented by a curve labeled CONVENTIONAL wind tunnel inFIG. 11) between the tips of the heat sink fins and the cover plate ofthe circuit pack housing. The clearance was 1.25 cm (worst case) and thepower dissipation per component was 16W. The data was taken at a fanvoltage of 32V. To have the same air velocity in the circuit packhousing, the pressure drops were matched for the cases where there wastip clearance and where there was no tip clearance.

A couple of points are noteworthy in FIG. 11. The thermal resistanceΘ_(ja) found for the situation where there was clearance between thetips of the heat sink fins and the cover plate does not exceed about1.6° C./W. This indicates that a heat sink cooled with large amount ofcoolant flow by-pass still performs remarkably well. Althoughsignificant improvement can be obtained by directing the flow throughslots in the cover plate, the performance is well within most designlimits.

The second point to note is component arrangement. In the staggered andin-line circuit pack configurations, the cover plate was solid and andthere was no tip clearance. The results show that component arrangementdoes not impair performance as long as components are not placed in thedirect wake of any of the other components, immediately down stream ofone another, such that flow through the heat sink is blocked orotherwise impaired. Comparison of Θ_(ja) for solid cover, staggered andin-line, in FIG. 11, further corroborates this point. Therefore, one canconclude that properly designed narrow channel heat sinks perform wellregardless of component layout on a circuit pack, provided directblockage is avoided.

METHOD OF OPTIMIZING HEAT SINK DIMENSIONS

A method of optimizing the fin thickness (τ) and the space between fins(S) of a narrow channel heat sink is presented. The total thermalimpedance, from heat sink base to inlet air, is the sum of the impedancefrom heat sink base to the local air stream plus the thermal impedancefrom the local air stream to inlet air:

    R.sub.total =R.sub.heatsink +R.sub.air

where, ##EQU1## where η is the fin efficiency, h is the heat transfercoefficient, and A_(s) is the surface area of the heat sink. ##EQU2##where ρ is the density of the cooling fluid, in this case air, A_(f) isflow area of the heat sink, V is the flow velocity, and C_(p) is thespecific heat of the coolant. In the above equations, the fin efficiency(η), the heat transfer coefficient (h), and the heat sink surface are(A_(s)) are given as follows: ##EQU3## where ##EQU4## and K_(s) is thefin conductivity, ##EQU5## where D≈2S,Nu is the Nusselt number, andK_(f) is the coolant conductivity. ##EQU6## where L is the length of theheat sink in the direction of cooling fluid flow, W is the width of theheat sink, and H is the height of a heat sink fin. The Nusselt number(Nu) for fully developed flow through a high aspect ratio rectangularchannel is assumed to be 8. Since the local air stream temperaturevaries through the channel along a streamline, the average thermalimpedance value for R_(air) has been chosen. The error introduced byusing the average instead of the log mean temperature difference issmall. The flow area (A_(f)) of the heat sink and the flow velocity (V)are given as follows: ##EQU7## where ΔP is the pressure drop through theheat sink and μ is the viscosity of the cooling fluid. Substitutingthese terms in the thermal impedance equation and introducing threedimensionless groups X=S/H, Y=τ/H, and K=K_(f) Nu/K_(s), the followingequation is obtained: ##EQU8## This equation can be optimized for thetwo independent variables (X) and (Y) in many ways. A graphical methodcan be used involving the plotting of the total thermal impedance as afunction of fin thickness parameter τ/H and channel spacing parameterS/H in a contour plot using a software package called SURFER availablefrom Golden Software, 807 14th Street, P.O. Box 281, Golden, Colo.80402. FIG. 12 shows such a plot for an air cooled aluminum heat sinkwith L=W=25 mm, H=12.5 mm, and ΔP=0.15 cm of water. The contour plotshows a minimum thermal impedance of 1.54° C./W at S/H=0.077 andτ/H=0.014, which corresponds to an optimum channel spacing of 0.96 mmand an optimum fin thickness of 0.18 mm. FIG. 13 shows the effect ofhigher coolant pressure and velocity with L=W=25 mm, H=12.5 mm, andΔP=0.5 cm of water. The contour plot shows a minimum thermal impedanceof 0.94° C./W at S/H=0.057 and τ/H=0.014 which corresponds to an optimumchannel spacing of 0.72 mm and an optimum fin thickness of 0.18 mm.

FIG. 14 is a contour plot for a heat sink like that of FIGS. 12 and 13,but for a coolant flow rate which creates a pressure difference ΔPacross the heat sink of 0.05 cm. H₂ O. The plot of FIG. 14 shows aminimum thermal impedance of 2.4° C./W at S/H=0.103 and τ/H=0.014 whichcorresponds to an optimum channel spacing of 1.287 mm. and an optimumfin thickness of 0.175 mm. FIG. 15 is a contour plot for a heat sinkanalogous to that of FIGS. 12-14, but for a coolant flow rate whichcreates a pressure difference of 1.5 cm. H₂ O. The plot of FIG. 15 showsa minimum thermal impedance of 2.4° C./W at S/H=0.045 and τ/H=0.015which corresponds to an optimum channel spacing of 0.563 mm. and anoptimum fin thickness of 0.188 mm. It may be said in light of theseresults that a most preferred range of channel width is about 0.5 mm.(S/H of about 0.04) to about 1.3 mm. (S/H of about 0.104) and a mostpreferred range of fin thickness is about 0.17 mm. (τ/H of about 0.0136)to about 0.19 mm. (τ/H of about 0.0152) for the heat sinks and pressuredrops considered in this detailed example.

Note that the thermal impedance increases rapidly for fin thicknessesand channel spacings less than optimum, and more slowly for finthicknesses and channel spacings greater than optimum. Due to productionvariations, it may thus be advantageous to design a heat sink with finthickness and channel spacing slightly larger than optimum.

SOME ALTERNATIVE EMBODIMENTS

FIG. 16 shows a heat sink in accordance with this invention which hasbeen integrated with a heat producing electronic component. The heatsink of FIG. 16 is created by forming a number of rectangular parallelchannels in a rectangular block of semiconductor material which issuitable for the fabrication of integrated circuits. The formation ofthe channels in the block of semiconductor material creates a series offins 12 extending from a base 14 having a predetermined height and apredetermined width depending upon the depths of the channels and thedistances separating the channels. The channels are formed so that thechannel width and fin thickness dimensions are optimized as describedabove. An integrated circuit 48 is formed in the base 14 of the heatsink by any technique of creating integrated circuits such asphotolithography.

FIG. 17 shows a heat sink which has been incorporated into anencapsulated or molded heat producing electronic component 50 such as ahybrid integrated circuit. The component 50 comprises a substratesupporting one or more heat producing electronic components which may beintegrated circuits or discrete components. The substrate also supportsa heat sink 52 dimensioned in accordance with the design procedures ofthis invention. The heat sink is attached in heat conductiverelationship with one or more heat producing electronic components. Thecomponent 50 also includes a molding or encapsulation material 54 whichcovers the components located on the substrate. A cavity 56 is formed inthe encapsulation or molding material 54 which allows cooling fluid flowto be directed through the channels of the heat sink 52 in the component50. In an alternative arrangement, the heat sink fins may extend outsidethe encapsulation material 54 and be cooled by coolant flow directedover the exterior of component 50.

CONCLUSION

A narrow channel heat sink has been described and its thermalperformance has been verified. The heat sink's performance is exemplaryfor a variety of flow directions. Tip clearance (flow bypass) may reducethe thermal performance of the heat sink, but the effect is notsignificant enough to prevent advantageous use of heat sinks inaccordance with this invention in conventional electronic coolingapplications. Component layout on the circuit board does notsignificantly affect thermal performance of the heat sink as long as theflow of cooling fluid through the heat sinks is not blocked. Narrowchannel heat sinks require only moderate pressure drops to achieve theimproved performance reported here. This is readily attained withstandard cooling fans. Narrow channel heat sinks can be readily used tocool high power electronic components. Components dissipating 20 W/cm²(based on heat source area) or higher showed maximum temperature risesof only about 38° C. for moderate forced air convection cooling.

We claim:
 1. A circuit pack, comprising:a housing; an inlet forreceiving cooling fluid into the housing; an outlet for exhaustingcooling fluid out of the housing; the inlet and outlet defining acooling fluid flow path through the housing; at least one heat producingelectronic device located in the housing; and a heat sink attached tothe at least one heat producing electronic device; the heat sinkcomprising: a block of heat conductive material having a front surface,a back surface, a top surface, and a unitary bottom surface; and aplurality of substantially continuous fins of predetermined thicknessand predetermined height formed in the top surface of the heat sink, thefins defining a plurality of continuous elongated channels each having apredetermined width continuously extending from the front surface to theback surface, the channels being oriented so that fluid flows throughthe heat sink along a path substantially parallel to the fluid flow paththrough the housing between the inlet and the outlet; the ratio of thepredetermined thickness to the predetermined height falling in the rangeof about 0.005 to about 0.055 and the ratio of the predetermined widthto the predetermined height falling in the range to about 0.03 to about0.13.
 2. The circuit pack of claim 1:in which the ratio of thepredetermined thickness to the predetermined height is from about 0.005to about 0.055; and in which the ratio of the predetermined width to thepredetermined height is from about 0.080 to about 0.130.
 3. The circuitpack of claim 1:in which the ratio of the predetermined thickness to thepredetermined height is from about 0.005 to about 0.055; and in whichthe ratio of the predetermined width to the predetermined height is fromabout 0.060 to about 0.110.
 4. The circuit pack of claim 1:in which theratio of the predetermined thickness to the predetermined height is fromabout 0.005 to about 0.055; and in which the ratio of the predeterminedwidth to the predetermined height is from about 0.040 to about 0.090. 5.The circuit pack of claim 1:in which the ratio of the predeterminedthickness to the predetermined height is from about 0.005 to about0.055; and in which the ratio of the predetermined width to thepredetermined height is from about 0.030 to about 0.080.
 6. The circuitpack of claim 1, which the ratio of the predetermined thickness to thepredetermined height out 0.014 and the ratio of the predetermined widthto the predetermined height is about 0.103.
 7. The circuit pack of claim1, in which the ratio of the predetermined thickness to thepredetermined height is about 0.014 and the ratio of the predeterminedwidth to the predetermined height is about 0.077.
 8. The circuit pack ofclaim 1, in which the ratio of the predetermined thickness to thepredetermined height is about 0.014 and the ratio of the predeterminedwidth to the predetermined height is about 0.057.
 9. The circuit pack ofclaim 1, in which the ratio of the predetermined thickness to thepredetermined height is about 0.015 and the ratio of the predeterminedwidth to the predetermined height is about 0.045.
 10. The circuit packof claim 1, in which the fin thickness is about 0.4 mm. and the channelwidth is about 1.1 mm.
 11. An apparatus, comprising:a plurality ofelectronic components; a base plate supporting the plurality ofelectronic components; a plurality of heat sinks, each in heatconductive relationship with one or more of the electronic components,each of the heat sinks comprising a block of heat conductive materialhaving a front surface, a back surface, a top surface, a unitary bottomsurface and a plurality of substantially continuous fins ofpredetermined thickness and predetermined height formed in the topsurface of the heat sink, the fins defining a plurality of continuouselongated channels each having a predetermined width continuouslyextending from the front surface to the back surface of the heat sink,the channels thereby confining fluid flow along longitudinal axes of theelongated channels between the front surface and the back surface of theheat sink; the ratio of the predetermined thickness to the predeterminedheight falling in the range of about 0.005 to about 0.055 and the ratioof the predetermined width to the predetermined height falling in therange of about 0.03 to about 0.13; a means for enclosing the heat sinkson the base plate comprising a top plate covering the heat sinks; and ameans for introducing cooling fluid into the enclosing means anddirecting the cooling fluid through the channels in the heat sinksparallel to the longitudinal axes of the channels.
 12. The apparatus ofclaim 11, in which the plurality of heat sinks comprises a row of heatsinks arranged along a line perpendicular to the predetermined directionof coolant flow in the means for enclosing.
 13. The apparatus of claim12, in which the heat sinks are staggered along a line parallel to thedirection of coolant flow in the means for enclosing.
 14. The apparatusof claim 11, further comprising a coolant entry slot in the top plate ofthe means for enclosing adjacent each of the heat sinks on the baseplate which permits entry of cooling fluid downwardly into the channelsof the heat sink associated with that coolant entry slot.
 15. Theapparatus of claim 11, in which the heat sinks comprise:a plurality ofrectangular fins, each having a predetermined thickness and apredetermined height; and at least one channel of a predetermined widthbetween the fins; in which a ratio of the predetermined thickness to thepredetermined height is from about 0.005 to about 0.055; and in which aratio of the predetermined width to the predetermined height is fromabout 0.03 to about 0.13.
 16. The apparatus of claim 11:in which theratio of the predetermined thickness to the predetermined height is fromabout 0.005 to about 0.055; and in which the ratio of the predeterminedwidth to the predetermined height is from about 0.080 to about 0.130.17. The apparatus of claim 11:in which the ratio of the predeterminedthickness to the predetermined height is from about 0.005 to about0.055; and in which the ratio of the predetermined width to thepredetermined height is from about 0.060 to about 0.110.
 18. Theapparatus of claim 11:in which the ratio of the predetermined thicknessto the predetermined height is from about 0.005 to about 0.055; and inwhich the ratio of the predetermined width to the predetermined heightis from about 0.040 to about 0.090.
 19. The apparatus of claim 11:inwhich the ratio of the predetermined thickness to the predeterminedheight is from about 0.005 to about 0.055; and in which the ratio of thepredetermined width to the predetermined height is from about 0.030 toabout 0.080.
 20. The apparatus of claim 11, in which the ratio of thepredetermined thickness to the predetermined height is about 0.014 andthe ratio of the predetermined width to the predetermined height isabout 0.103.
 21. The apparatus of claim 11, in which the ratio of thepredetermined thickness to the predetermined height is about 0.014 andthe ratio of the predetermined width to the predetermined height isabout 0.077.
 22. The apparatus of claim 11, in which the ratio of thepredetermined thickness to the predetermined height is about 0.014 andthe ratio of the predetermined width to the predetermined height isabout 0.057.
 23. The apparatus of claim 11, in which the ratio of thepredetermined thickness to the predetermined height is about 0.015 andthe ratio of the predetermined width to the predetermined height isabout 0.045.
 24. The apparatus of claim 11, in which the fin thicknessis about 0.4 mm. and the channel width is about 1.1 mm.